关键词： Molecular dynamics (MD) simulations   Mechanical properties   Uniformly/randomly distributed nanopores   Failure mechanism  

摘要： Binary compound Mg2Sn has received close attention for the thermoelectric energy conversion application, due to the abundance of constituent elements and the non-toxicity in the working condition. The introduction of nano-scale pores into crystalline Mg2Sn can significantly reduce the lattice thermal conductivity, while the nanopores might weaken the mechanical properties that are important for the commercial applications of Mg2Sn. To determine the effect of nanopores on mechanical behavior of Mg2Sn, we investigate the stress responses of tensile and shear deformations in uniformly and randomly distributed nanoporous bulks by using molecular dynamics (MD) simulations. We find that nanopores reduce the mechanical properties such as ultimate stress, fracture strain and elastic modulus of crystalline Mg2Sn. Furthermore, mechanical properties of randomly distributed nanoporous Mg2Sn are poorer than those of uniformly distributed nanoporous Mg2Sn. Nanopores lead to a reduction both in ultimate stress and elastic modulus. Meanwhile, the strength and the stiffness are closed related to porosity rather than radius of pores. In the case of tensile loading, the crack of (0 0 1)/<1 1 0> tilt orientations in the vicinity of nanopores results in zigzag breakage surfaces. As for shear loading, the atomic concentration, pore coalescence and shear band explore the fracture mechanism. Our results provide new perspectives of failure mechanism of nanoporous Mg2Sn under different loading methods.

关键词： Surface area to volume ratio   Computational methods   Tensile properties   Nanowires   Surface energy   Molecular dynamics   Crystal orientation   Mechanical properties   Elastic modulus  

摘要： A primary challenge to use silicon nanowires as a truly potential building block in nanoscale devices is the implementation of scale effects into operational performance. Therefore, surface stress effects-as a direct result of size reduction-on transport properties became a major field of study. Previous computational simulations have focused so far on geometrical parameters with symmetrical cross sections, while silicon nanowires with nonsymmetrical cross sections are the major result of top-down fabrication techniques. A recent study has drawn a new aspect on the role played by the surface stress with a torsional profile on silicon nanowires to address the existing controversy from experimental and computational studies. Motivated by its success, the implications of this surface stress profile on the tensile properties of silicon nanowires are studied through molecular dynamics simulations. Deformation associated with the surface stress is computed for different length-to-thickness and width-to-thickness ratios. Then, tensile properties are investigated for a constant strain rate. Atomic calculations are carried out on silicon nanowires along the < 100 > crystal orientation for fixed-fixed and fixed-free boundary conditions. A combination of compressive uniaxial surface stress and torsional surface stress contributes to the mechanical behavior of silicon nanowires. A transition on elastic properties is obtained through changing the cross section from square to rectangular configuration. Further to addressing the controversy regarding the contribution of the surface stress on the mechanical properties, limits associated with available analytical approaches are highlighted for silicon nanowires. Published under license by AIP Publishing.

关键词： cleaner preparation   dielectric properties   nanocomposites   relaxation dynamics  

摘要： Traditional polymer composite preparation techniques often employ organic solvents, which can damage the environment, to disperse inorganic fillers. In this article, classic nanocomposites with poly(vinylidene fluoride) (PVDF) polymer matrices and BaTiO3 nanoparticle (BTP) fillers were created by a clean method combining planetary ball milling with an ultracentrifugal mill and then hot pressed into thin films. The microstructures, properties and relaxation dynamics of the thin films were characterized and analyzed. Scanning electron microscopy results demonstrated that BTP was homogeneously dispersed in the PVDF matrix. The thermal, mechanical, and dielectric properties were comparable to those of composite films prepared by solution mixing. Dielectric analysis revealed that the dielectric constant of the thin films reached 14 (10(4) Hz) when the volume fraction of BTP was 30%;however, the dielectric loss was 0.1 (10(4) Hz). Additionally, the dielectric loss spectra fitted with the Havriliak-Negami (H-N) and Vogel Fulcher equations were employed to analyze the relaxation dynamics of the nanocomposites. (c) 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47254.

关键词： 计算机辅助设计   自锁缓降器   动力学仿真   强度校核  

摘要： 为了提高矿用重型机械的安全性,设计了提升擒纵式自锁缓降器,对缓降器的可靠性和动力学性能进行了理论分析。利用CAD和UG软件设计了缓降器的三维结构,并用Adams软件进行了动力学性能仿真,分别获得了主传动轴的角速度及碰撞接触力,并对擒纵轮进行了应力分析。最后,对试验样机的工作性能进行了测试。研究结果表明,擒纵式缓降器机械结构在下落过程中不会出现打滑现象,强度和动力特性满足使用要求;通过试验,验证了仿真结果的正确性。研究结果对于缓降器的优化设计具有较好的指导作用。

关键词： 右美托咪定   剂量选择   ICU机械通气   镇静   血流动力学  

摘要： 目的探究不同剂量右美托咪定对ICU机械通气患者镇静效果和血流动力学的影响。方法选取我院ICU接受机械通气治疗的患者120例作为研究对象,均给予右美托咪定,根据剂量的选择分为大剂量组(60例)和小剂量组(60例)。小剂量组负荷剂量和维持剂量分别为0.5μg/kg、0.2μg/(kg·h),大剂量组负荷剂量和维持剂量分别为1.0μg/kg、0.4μg/(kg·h),比较两组的应用效果。结果在T0时间点时,小剂量组和大剂量组SBP、DBP、HR差异无统计学意义(P>0.05);在T1、T2、T3时间点时,大剂量组和小剂量组差异有统计学意义(P<0.05);大剂量组在T0、T1、T2、T3等时间点镇静评分方面,均要高于小剂量组,但差异并无统计学意义(P>0.05);在尿量、呼吸率和血氧饱和度等指标的比较中,小剂量组和大剂量组除了T0时间点差异无统计学意义外,T1、T2、T差异均有统计学意义(P<0.05)。结论右美托咪定剂量越大,血流动力学的变化幅度越大,对镇静效果的影响不大,右美托咪定剂量越小,血流动力学越稳定,值得推广。

关键词： 发电厂   机械动力设备   设备检修  

摘要： 作为重要的能源,电力供应与整个国民经济发展以及社会的正常运转都有着直接、紧密的联系,在电力结构中,火力发电作为我国主要的电力来源之一,在保障日益增长的电能需求方面发挥着及其重要的作用。随着社会需求量的增加,火力发电厂运行负荷持续上升,这也就加重了电厂动力设备的负担。为确保火电厂动力设备稳定运行,加强火电厂动力设备的检修与维护就显得十分必要。本文总结了火力发电行业的现存问题,分析火力发电动力设备的常见故障,着重探讨了动力设备的检修与维护。

关键词： 柔性多体动力学   机械臂   结构优化  

摘要： 从柔性多体动力学对机械臂结构影响出发,分析了基于柔性多体动力学的机械臂结构优化设计方式,旨在为柔性多体动力学在机械臂结构优化设计中的应用奠定技术基础,以此提高机械臂整体动力学性能,进而提高机器人载重、自重比和末端操作精度,提升机械臂应用效果和技术含量。

关键词： 机械通气   镇静   右美托咪定   不同剂量   血流动力学  

摘要： 目的观察机械通气患者应用不同剂量右美托咪定的临床效果。方法选择60例机械通气患者(至少6 h以上),随机分为为D1组、D2组、D3组,分别给予不同剂量的右美托咪定,给药过程中在患者感觉疼痛时立即使用瑞芬太尼补救,维持患者镇痛状态。观察并记录其用药前后各个时间点的收缩压、舒张压及心率变化。结果随着时间的延长,心率(HR)、收缩压(SBP)、舒张压(DBP)各参数组内差异均有统计学意义(P<0.05),D2组的HR、SBP、DBP优于其他2组(P<0.05)。结论机械通气患者给予0.25μg/kg负荷剂量的右美托咪定后镇静良好,血流动力学稳定。

关键词： phaqraphene   mechanical   properties   crack   propagation   molecular   dynamics   thermal   effects  

摘要： Phagraphene is a very attractive two-dimensional (2D) full carbon allotrope with very interesting mechanical, electronic, optical, and thermal properties. The objective of this study is to investigate the mechanical properties of this new graphene like 2D material. In this work, mechanical properties of phagraphene have been studied not only in the defect-free form, but also with the critical defect of line cracks, using the classical molecular dynamics simulations. Our study shows that the pristine phagraphene in zigzag direction experience a ductile behavior under uniaxial tensile loading and the nanosheet in this direction are less sensitive to temperature changes as compared to the armchair direction. We studied different crack lengths to explore the in fluence of defects on the mechanical properties of phagraphene. We also investigated the temperature effect on the mechanical properties of pristine and defective phagraphene. Our classical atomistic simulation results confirm that larger cracks can reduce the strength of the phagraphene. Moreover, it was shown the temperature has a considerable weakening effect on the tensile strength of phagraphene. The results of this study may be useful for the design of nano-devices using the phagraphene.

关键词： 慢性阻塞性肺疾病   急性左心衰竭   血流动力学   通气反应   急性加重   通气治疗   pulmonary   患者  

摘要： 随着时代的发展,资源消耗迅速,大气污染严重,吸烟人群众多,人口老龄化加剧,慢性阻塞性肺疾病(chronic obstructive pulmonary disease,COPD)为气流不完全受限的肺部疾病,其特点是常见、多发,严重威胁人们的身心健康,且有极高的致死率,目前尚未有有效的彻底根除的治疗方法。